Memory device using movement of protons

ABSTRACT

An electrically written memory element utilizing the motion of protons within a dielectric layer surrounded by layers on either side to confine the protons within the dielectric layer with electrode means attached to the surrounding layers to change the spatial position of the protons within the dielectric layer. The device is preferably constructed as a silicon-silicon dioxide-silicon layered structure with the protons being introduced to the structure laterally through the exposed edges of the silicon dioxide layer during a high temperature anneal in an atmosphere containing hydrogen gas. The device operates at low power, is preferably nonvolatile, is radiation tolerant, and is compatible with convention silicon MOS processing for integration with other microelectronic elements on the same silicon substrate. With the addition of an optically active layer, the memory element becomes an electrically written, optically read optical memory element.

This invention was made with Government support under ContractDE-AC04-94AL85000 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

This invention relates to a new electronic memory mechanism involvingthe movement of protons (hereinafter hydrogenous ions) within adielectric layer. The memory mechanism may manifest as a static randomaccess memory, a dynamic random access memory, a nonvolatile memory, anoptical memory, or as a flat panel display. The memory elements are mostreadily constructed in silicon-based structures.

There is a wide variety of structures that can act as memory elements.Each has its own advantages and disadvantages. The ubiquitous dynamicrandom access memories (DRAM) have high capacities, relatively low cost,and fast access times but are subject to upsets from radiation and othermechanisms and require refresh and rewrite circuits. Static RAMs requiremore circuitry and also are subject to upsets. Flash and silicon-nitridebased (SONOS) nonvolatile memories have good access times and endurancebut require high (12-15V) programming voltages. EEPROM nonvolatilememories have similar characteristics. Ferroelectric memories areincompatible with silicon processing but are otherwise attractivenonvolatile memories. What is needed is a low cost, silicon compatible,low voltage write/read memory that is radiation tolerant and nonvolatilewith reasonable write speeds and endurance. Unfortunately no such deviceexists in the art at this time.

SUMMARY OF THE INVENTION

The basic mechanism exploited herein to form the memory elements of thisinvention is the movement of ions containing a single proton (hydrogen+,deuterium+, and tritium+--hereinafter hydrogenous ions) within adielectric layer contained between two semiconducting or conductinglayers that are impermeable to the ions. An electric field is appliedreversibly across the dielectric layer to change the spatial position ofthe hydrogenous ions within the dielectric layer. The position of thehydrogenous ions remains the same and endures unless intentionally movedby the application of a reversed electric field across the dielectriclayer. Since the protons create their own positive electric field, theirspatial position within the dielectric layer can be used to form amemory element.

When implemented as a microelectronic memory, silicon dioxide is apreferred material for the dielectric layer. The hydrogenous ions areintroduced into the dielectric layer either by a high temperature annealin a hydrogenous gas atmosphere or by implanting hydrogenous ionsdirectly into the dielectric layer. The layers above and below thedielectric layer comprise a material that is highly imperious to themigration of the hydrogenous ions with poly or single crystallinesilicon as a preferred choice. Electrodes are associated with the layersabove and below the dielectric layer to create the electric field tochange the spatial position of the hydrogenous ions. The state of thememory element is determined by sensing the conductivity of either ofthe layers adjacent to the dielectric. If the hydrogenous ions have beenmoved next to the interface between the dielectric layer and theadjacent semiconducting layer, the positive electric field from the ionswill attract electrons from the adjacent semiconducting layer to theinterface thereby changing the conductivity. When realized with silicondioxide as the dielectric layer, the memory element is nonvolatile. Whenother dielectric materials are employed, the memory element can be anonvolatile memory or a DRAM that is radiation tolerant.

When implemented as an optical memory element, an additional componentis added. This is an electro-sensitive optical layer that is disposedbetween the upper layer and the top electrode which is now made of atransparent electrode material. The optical layer is sensitive to thespatial position of the ions and their associated electric field. Anoptical characteristic of the layer changes in response to the presenceor absence of this field. The element would typically be formed as partof a large array of elements, each of which could be individually readby light passing completely through the element or reflecting off theside with the optical layer. The array of elements could also beconfigured as a flat panel display.

One preferred embodiment of the invention is a memory elementcomprising: a first dielectric layer containing mobile hydrogenous ions;a lower layer below the first layer of a material highly impervious tomigration of hydrogenous ions therethrough; an upper layer above thefirst layer of a material highly impervious to migration of hydrogenousions therethrough; and means to change the spatial position of thehydrogenous ions and the electric field created thereby within the firstlayer. This embodiment may additionally comprise means to sense theposition of the electric field with the first layer. This means to sensethe position of the electric field may comprise either one of means tosense the conductivity of either the upper or lower layer adjacent tothe first layer or means to sense an optical property in an additionallayer of a material that exhibits a change in the optical property inresponse to a change in the proximity of the electric field in the firstlayer.

Another embodiment of the invention is a microelectronic memory elementcomprising: a silicon substrate layer; a first layer containing mobilehydrogenous ions; a second silicon layer above the first layer; andmeans to change the conductivity of at least one of the silicon layersby changing the spatial position of the hydrogenous ions in the firstlayer. In this embodiment, the first layer may comprise a dielectricwhere the dielectric is silicon oxide, barium strontium titanate,tantalum oxide or nitrided oxide. This embodiment may additionallyinclude a material highly impervious to migration of hydrogenous ionstherethrough surrounding the first layer. Such material may be nitrides,nitrided oxide, silicon nitride, polysilicon or monocrystalline silicon.This memory element may further include interconnections to othernon-memory microelectronic elements formed on the same siliconsubstrate. This memory element may further include cracking siteslocated in the first layer for creation of the hydrogenous ions from thecracking of the molecules of a hydrogenous gas wherein the hydrogenousions are selected from the group consisting of hydrogen ions, deuteriumions, tritium ions, and combinations thereof. This memory element mayalso be non-volatile. In this embodiment the means to changeconductivity may comprise substrate electrode means and gate electrodemeans, said gate electrode means being located above the second siliconlayer, with the substrate electrode means and the gate electrode meansbeing connected to reversible voltage bias means which drives thehydrogenous ions towards the upper surface of the first layer when thesubstrate electrode means is biased positively relative to the gateelectrode means and drives the hydrogenous ions towards the lowersurface of the first layer when the gate electrode means is biasedpositively relative to the substrate electrode means. Finally, thisembodiment may further comprise an intermediate silicon layerimmediately above the first layer and an additional layer containingmobile hydrogenous ions above the intermediate silicon layer and belowthe second silicon layer and an additional component in the means tochange the conductivity of the second silicon layer to change thespatial position of the hydrogenous ions in the additional layer to forma multi-state memory element.

In yet another embodiment the optical memory element may comprise: afirst dielectric layer containing mobile hydrogenous ions; a lowerlayer, below the first layer, of a material highly impervious tomigration of hydrogenous ions therethrough; an upper layer, above thefirst layer, of a material highly impervious to migration of hydrogenousions therethrough; an electro-sensitive optical layer above the upperlayer in which an optical property changes in response to a change inthe spatial position of the hydrogenous ions in the first layer; and anelectrode means associated with the lower layer and a transparentelectrode means located above the electro-sensitive optical layer, withboth electrode means connected to reversible voltage bias means tochange the spatial position of the hydrogenous ions within the firstlayer. In this embodiment the optical property may be polarization orabsorption, and the optical memory element itself may be transparent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross sectional view of one embodiment of the electronicmemory element formed as a pseudo (Ψ) MOSFET with the hydrogenous ionsat the upper boundary of the oxide layer, and FIG. 1B is a simplifiedgraph of the current voltage (I-V) characteristic of the element.

FIG. 2A is a cross sectional view of the embodiment of the electronicmemory element shown in FIG. 1A with the hydrogenous ions at the lowerboundary of the oxide layer, and FIG. 2B is a simplified graph of theI-V characteristic of the element.

FIG. 3A is a graph of the I-V characteristic of a SIMOX Ψ-MOSFETstructure, and FIG. 3B is a graph of the C-V characteristic of an MOScapacitor, as shown in the respective inserts.

FIGS. 4A and 4B show, respectively, drain current decay and a crosssectional schematic representation of the migration of the hydrogenousions causing the decay.

FIG. 5 is a graph showing initial rate of decay of drain current as afunction of gate bias. Each data point represents the initial slope ofthe I-t curves of FIG. 4A. Different I-t curves were obtained by firstapplying a +60 V substrate bias for 5 minutes to accumulate the ions atthe interface between the oxide layer and the upper Si layer, followedby switching to different negative substrate biases. The dashed line isa y=ax² least squares fit to the data (a=0.002).

FIG. 6A is an Arrhenius plot of the initial rate of decay of thenormalized drain current. The data were taken as in FIG. 5 by firstapplying a +60 V substrate bias for 5 minutes, but now followed byswitching to -10 V substrate bias at different substrate temperatures.FIG. 6B shows the normalized decay of the drain current vs. time at 73°C.

FIG. 7 is a cross sectional view of another embodiment of the electronicmemory element, here an embedded memory element for use in conjunctionwith non-memory elements on the same silicon substrate.

FIG. 8 is a graph of the measured I-V characteristic for the element ofFIG. 7.

FIG. 9 is a graph showing the retention characteristic of the element ofFIG. 7 at room temperature.

FIG. 10 is a graph showing the retention characteristic of the elementof FIG. 7 at high temperature.

FIGS. 11A and 11B are endurance and radiation tolerance response graphsfor a memory element of this invention and a conventional electronicmemory element, respectively.

FIG. 12 is a cross sectional view of an optical memory element usedreflectively.

FIG. 13 is a cross sectional view of an optical memory element usedtransmissively.

DETAILED DESCRIPTION OF THE INVENTION

The effects of hydrogen ions in microelectronic devices have beenstudied for some time. Metal-oxide-semiconductor (MOS) devices whenexposed to ionizing radiation or hot carrier stress degrade due to thecreation of electrically active defects at the oxide-semiconductorinterface. Many studies suggest that ionic hydrogen plays a key role inthis degradation. In the course of studying these problems, theimportant discovery of the hystersis of the current-voltage (IV)characteristics in a dielectric layer containing such protons andsurrounded by conducting or semiconducting layers was made by theinventors herein. The initial discovery was made in the course ofinvestigating the silicon-on-insulator (SOI) material system but hassince been expanded to more conventional silicon processing systems.

As was mentioned above, the fundamental proton displacement mechanism isapplicable to electronic memory and also to optical memory that isoptically read but electrically written. The electronic memoryembodiment will be discussed first.

Electronic Memory

The mechanism embodied in an electronic memory is perhaps most readilyunderstood by study of FIGS. 1A, 1B, 2A and 2B. FIG. 1A is a simpleembodiment of the electronic memory with the oxide layer 10 surroundedby the silicon substrate 12 below and the upper p-Si layer 14 above. Agate electrode 18 is attached to the bottom of the substrate, and source20 and drain 22 electrodes are attached to n+ regions of the upper layer14. The mobile hydrogenous ions 16 are at the top of the oxide layer.This spatial position of the ions produces the `ON` condition shown inthe simplified I-V curve in FIG. 1B. The presence of the positivelycharged ions 16 attracts the mobile electrons in the p region of theupper layer 14 to the interface between layers 10 and 14, therebycreating a conducting channel at the base of the upper layer 14 so thatcurrent will easily pass between the source and drain electrodes. FIG.2A shows the same device with the hydrogenous ions 16 now moved to thebottom of the oxide layer 10. In this position the electric fieldcreated by the ions is no longer strong enough to create the conductivechannel in the upper layer 14. This produces the `OFF` condition shownin the I-V curve in FIG. 2B. The ions 16 are moved by placing thevoltage biases shown on the gate electrode 18 as shown in FIGS. 1A and2A. These devices could also be fabricated with p+ regions at the sourceand drain electrodes.

The initial devices created to demonstrate the invention were formed inSi/SiO₂ /Si structures. H+ (D+) ions were introduced into the buriedSiO₂ layer by annealing the structure in a hydrogen (deuterium)containing atmosphere. The spatial position of the ions in the oxidelayer was monitored by high frequency capacitance voltage and currentvoltage measurements (FIGS. 3A and 3B). The reversibility and transientbehavior of the proton and deuteron migration inside the oxide layer wasstudied as a function of gate bias across the oxide at differenttemperatures and demonstrated that the ions created by the introductionof the hydrogenous gas were responsible for the hysteresis effectsobserved.

Three different types of Si/SiO₂ /Si materials were investigated.Separation by the implantation of oxygen (SIMOX) samples were formed byimplanting a p-type Si(100) substrate with 190-keV O+ ions to a dose of1.8×10¹⁸ cm⁻² followed by a subsequent anneal at 1320° C. in Ar+1% O₂.This resulted in a 200-nm monocrystalline Si (c-Si) layer on top of a400-nm buried oxide layer, so the oxide layer is sandwiched between twoc-Si layers. Zone-melt-recrystallization (ZMR) SOI wafers, p-type (100)Si material, with a 330-nm monocrystalline Si layer on top of a1-μm-thick buried oxide layer, were also investigated. Apart from thesetwo SOI materials we also studied a standard thermal SiO₂ (40-nm thick)capped with an undoped poly-Si layer (1-μm thick),chemical-vapor-deposited and annealed at 1200° C. for 2 h in Ar+1% O₂.

Part of the top Si layer of the wafers was etched in a HNO₃ --CH₃COOH--HF mixture through a mask, leaving behind isolated rectangularstrips (2 mm by 9 mm) of top Si layer. This was done to reduce leakagecurrents at the substrate edges and through the buried oxide layerduring the subsequent point contactpseudo-metal-oxide-semiconductor-field effect-transistor (Ψ-MOSFET)measurements, and to facilitate lateral diffusion of hydrogen(deuterium) into the buried oxide during the forming gas anneal. Thislateral diffusion is crucial because the amount of hydrogen diffusingthrough the top c-Si layer into the buried oxide is negligible, due tothe very low solubility of hydrogen in Si.

Forming-gas (FG) N₂ :H₂ ; 95:5 or N₂ :D₂ ; 95:5 (by volume, 99.999%pure)! and nitrogen (99.999% pure) anneal treatments were performedusing a flow through a quartz tube inserted into a tube furnace between300° and 900° C. for 30 min. For the capacitance-voltage (C-V)measurements, the Si strips were etched off using KOH. C-V measurementson the Si/SiO₂ structures resulting from the KOH etch were made at 1 MHzwith a mercury probe or Al dots (≈1 mm² contact area) forming gatecontacts. Current-voltage (I-V) measurements on the Si/SiO₂ /Sistructures were performed using the point-contact Ψ-MOSFET technique.The buried oxide plays the role of the gate dielectric and the top Silayer represents the transistor body. Two tips of a standard four pointprobe are placed on the top Si layer to form the source and drain pointcontacts, while the gate voltage is applied to the back of the Sisubstrate.

FIG. 3A shows the hysteretic behavior of the I-V curves on SIMOX afterit received a 550° C. FG anneal. Similar features were observed in theZMR and the poly-Si capped thermal oxide, and occur over a FG-annealtemperature range from 500° to 800° C. Curve 1 was recorded with adecreasing gate (substrate) bias (from positive to negative) after thebias was kept constant at the initial value (+40 V) for 5 minutes. Curve2 was subsequently recorded using the opposite gate voltage sweepdirection (from negative to positive) after holding the gate bias at theinitial value (-70 V) for 5 min. The "hysteretic" behavior is evidencedby the fact that the original I-V curve (curve 1) was not retraced byreversing the voltage sweep direction (curve 2). Curve 3 was recordedafter curve 2 using the same procedure described for curve 1, showingthe reversibility of the process.

In general, charge in the oxide will cause the I-V curve to shift alongthe voltage axis. This voltage shift (ΔV) is proportional to the chargedensity and depends on its spatial distribution in the SiO₂ layer:##EQU1## where e is the electronic charge, d is the oxide thickness, εis the oxide dielectric constant, and ρ(x) is the local charge densityin the dielectric. For the MOS capacitors, the variable "x" is thedistance between the local oxide charge r(x) and the metal gate; for theΨ-MOSFET, x is the distance between the charge and the substrate-Si/SiO₂interface. As can be seen from the integral in Eq. 1, ΔV is maximizedfor the Ψ-MOSFET if the charge is located near the top-Si/SiO₂ interface(x≈d) and is minimized if it is located near the substrate-Si/SiO₂interface (x≈0). The negative voltage shifts ΔV in the I-V plots arecaused by positive charges in the buried SiO₂ (areal density≈2×10¹²cm⁻²). The observed hysteretic behavior is the result of an electricfield induced migration of a charged ionic species from one Si/SiO₂interface to the other. This type of behavior was not observed afterannealing in Ar or N₂, showing that the presence of hydrogen in theanneal ambient triggers the hysteretic behavior; it is not due to mobileion contamination.

If the top Si layer is removed and an MOS capacitor is formed, it isobserved that the charged species escape the SiO₂ dielectric through themetal gate under negative gate bias. This further demonstrates that themobile species involved are positively charged. The crucial role of thehydrogen anneal step to trigger the hysteretic effect, and the muchhigher solubility of hydrogen and reaction in metal capacitor gates suchas Al or Hg as compared to Si, collectively suggest that the mobilecharge is H⁺.

The transient behavior of the field induced charge migration from anaccumulated interfacial layer into the bulk of the buried SiO₂ layer wasanalyzed as a function of different gate voltages (V_(G)) andtemperatures. This was done by first applying a positive V_(G) to thesubstrate of Si/SiO₂ /Si device structures, which causes positive ionaccumulation in a thin layer at the top-Si/SiO₂ interface (areal densityQ_(H) ⁺). After this, the polarity of V_(G) was reversed, forcing theions to migrate away from the accumulated layer into the bulk of theSiO₂. The exodus of ions away from the charge layer accumulated at thetop-Si/SiO₂ interface, induced by the applied V_(G), is well describedas space-charge-limited current. Space charge limited current flows whenmobile charge carriers are injected into an insulator, where no othercharges are present. Solving Poisson's equation for this problem yields:##EQU2## where ##EQU3## is the rate of decrease in the charge densityaccumulated at the top-Si/SiO₂ interface, J_(H) ⁺ is the space chargelimited ionic current density in the SiO₂ layer, and μH⁺ is the ionicmobility.

When the positive ionic charge, Q_(H) ⁺, is accumulated at thetop-Si/SiO₂ interface in a Ψ-MOSFET structure it will induce a negativethreshold voltage shift, ΔV, in the transistor characteristics. As aresult, an inversion channel is formed in the p-type top Si layer at thetop-Si/SiO₂ interface, and a drain current (I_(D)) will flow in theΨ-MOSFET device. We will assume a linear relationship between Q_(H) ⁺and the resulting threshold voltage shift ΔV at the top-Si/buried-SiO₂interface at any time during the field induced ionic drift. This is onlyan approximation since the exact expression for the threshold voltageshift involves integration over the entire oxide layer as described inEq. 1. We will also assume a linear relationship between ΔV and theΨ-MOSFET drain current I_(D) at any gate voltage V_(G) smaller than thevoltage shift ΔV. As can be seen from FIG. 3A this appears to be areasonable approximation. Consequently, the decrease in Q_(H) ⁺ as afunction of time after switching the gate bias polarity, ##EQU4## can bemeasured as a decrease in I_(D) over time ##EQU5## FIG. 2(a) shows thedecay of I_(D) vs. time in a Ψ-MOSFET device on FG-annealed ZMRmaterial. The positive ions were first drawn to the top-Si/SiO₂interface with a V_(G) =+60 V substrate bias, followed by switching toV_(G) =-10 V and maintaining the substrate at 73° C. The schematic inFIG. 4B shows an SOI cross section, with the evolution of the conductionelectrons in the top-Si inversion layer and the ionic distribution, onthe same time scale as FIG. 4A. The initial rate of decay of theΨ-MOSFET drain current ##EQU6## obtained from the I_(D) vs. t curvessuch as the one shown in FIG. 4A was also analyzed. The V_(G) dependencewas measured by switching to different negative V_(G) after firstaccumulating the ions at the top-Si/SiO₂ interface. It was found thatthe electric field dependence of the initial decay rate is welldescribed by a square-law behavior as anticipated from Eq. 2. It shouldbe noted that the relatively high voltage employed and the relativelylong time scale are a function of the relatively thick oxide layertested here.

To first order, the ionic mobility in Eq. 2 is governed by a thermallyactivated hopping process and is given by: ##EQU7## where q is the ioniccharge, a is the intersite hopping distance, E_(a) is the activationenergy, k_(B) is Boltzmann's constant and T is the absolute temperature.The parameter ##EQU8## is the attempt to escape frequency, in which Φ"is the second derivative of the function describing the potential-wellof the hopping site, and m is the mass of the ionic species. Hoppingsimply implies that the ion moves through the lattice by consecutivejumps (hops) from one site to another, and that the ion must have enoughenergy (>E_(a)) before the jump can occur with any reasonableprobability.

To determine E_(a) for H⁺ motion, the initial rate of decay of theΨ-MOSFET drain current ##EQU9## in FG-annealed ZMR devices was againmeasured in the same way as described above, except that the temperaturewas varied at a fixed V_(G) (-10 V). FIG. 5 shows the Arrhenius plot of##EQU10## vs. T⁻¹ (see Eqs. 2 and 3) measured on Si/SiO₂ /Si (ZMR)structures that were hydrogenated or deuterated at 600° C. From thisplot, we deduce an activation energy E_(a) ≈0.8 eV for the ionicmobility for both the hydrogenated and deuterated buried SiO₂. Thisvalue is in good agreement with the diffusion activation energy of H⁺ instandard thermal SiO₂ thin films. The difference in absolute valuesbetween the two curves in FIG. 6A and 6B results from a difference inthe prefactor of Eq. 3 of 1.5, again in good agreement with thetheoretically expected difference of ##EQU11##

This observed hysteretic behavior can be utilized in a non-volatileNVFET memory device. An n-channel transistor can be changed to "normallyon" or "normally off" by applying a positive or negative gate(substrate) bias which will drift the protons to the top Si/SiO₂ orsubstrate Si/SiO₂ interface, respectively. For a memory device this canbe interpreted as writing the device to a bit state "1" or "0",respectively. To read the device, the zero bias drain current I₀ issimply measured (high current then corresponds to logic state "1", lowcurrent to "0"), as visualized in FIGS. 1, 2, 3 and 4. In contrast withthe instabilities that are usually associated with mobile ions in SiO₂,charge retention experiments on this device show that once written to aspecific state, the device remains in that state for over 10⁴ s whileheated to 200° C. Furthermore, fatigue experiments performed on thesestructures show that the device easily endures over 10⁴ write-erasecycles without any degradation.

For memory devices, a short write time is desirable. For the device usedin FIG. 4 the write time would be about 100 s. It follows from Eq. 2that the device speed is proportional to d⁻³, where d is the buriedoxide thickness. For devices fabricated using the poly-Si-capped 40-nmthermal oxide substrate as shown in FIG. 7, a write time of about 30 mswas observed at room temperature, in good agreement with the d⁻³dependence predicted from theory. Write times as fast as 1 ms can beexpected for 10-nm thermal oxides. Even more significantly, the latterresult demonstrates that these non-volatile field effect transistor(NVFET) memory devices do not require SOI substrates, but can befabricated using thermal oxides capped with a poly-Si layer, a standardprocedure in Si-MOS processing.

Turning to FIG. 7 for more detail on this alternative device, a p-typelayer 42 is formed on a silicon substrate 40. An oxide layer 44, whichcan be either a thermal oxide layer or a deposited oxide layer, isformed on top of the p-type layer 42 and capped by a polycrystallinesilicon layer 46. The hydrogenous ions are introduced into the oxidelayer at this point, either by annealing in a hydrogenous gas atmosphereor by ion implantation. The n+ drain 48 and source 50 regions areformed, and finally the various electrodes are formed. To write thedevice, normally substrate 41, source 51, and drain 49 electrodes wouldbe connected to ground with the gate electrode being biased positivelyor negatively to change the spatial position of the hydrogenous ions.FIG. 8 is a graph showing the I-V characteristic of the device of FIG.7, where the ions were formed in the oxide layer by a 30 minute annealat 550° C. in forming gas. This device could also be fabricated with p+regions about the source and drain electrodes.

This NVFET device has potential advantages over state-of-the-artnon-volatile memory technologies such as Flash and EEPROM. While itsspeed, retention and lifetime performance are certainly competitive withthese existing technologies, it is simpler in design, requires fewerprocessing steps, operates at much lower voltages.

Finally, we discuss the mechanism causing the incorporation of mobile H⁺ions in the buried SiO₂ layer of these Si/SiO₂ /Si materials during theFG anneal. It is known that the high-temperature (1200°-1325° C.)formation anneal step creates neutral O vacancies (Si--Si bonds) in theburied oxide via O out-diffusion from the SiO₂ into the top andsubstrate Si layers. Because these strained Si--Si bonds can act as H₂cracking sites in the buried SiO₂, they are catalyst sites for thegeneration of protons at elevated temperature. Since the solubility ofhydrogen species in c-Si is low, once formed, the H⁺ is largely"imprisoned" in the buried SiO₂ layer, sandwiched between the twoencapsulating Si layers, i.e., the interfaces form a diffusion barrierfor hydrogen. While there seems to be a peculiar affinity between SiO₂and the hydrogenous ions that provides the excellent characteristics fornonvolatile memory, the scope of the invention is intended to encompassany material which provides similar behavior with hydrogenous ions.

These cracking sites form naturally by the high temperature annealingprocess as the O from the silicon dioxide structures in the oxidediffuse across to the pure silicon regions in the layers on either side.As the O migrates away, Si--Si bonds are created in the oxide layersthat form the cracking sites. However, these Si--Si bonds may also becreated deeper within the oxide layer by implanting Si ions into theoxide layer to form the cracking sites. Once the cracking sites haveformed, one should avoid further prolonged long duration hightemperature processing steps for the devices. Short duration processessuch as Al metallizations at 450° C. or lower are permissible, buthigher temperatures should be avoided.

The silicon layers above and below are essentially impermeable to theseions. However, there may be some devices that require that a layer otherthan silicon be emplaced next to the central dielectric layer. Thismaterial must also act to confine the hydrogenous ions within thedielectric layer. Suitable materials include, but are not limited to,nitrides, nitrided oxide, silicon nitride, and polycrystalline andmonocrystalline silicon. Once the structure has been annealed and theions created, the ions tend to remain within the oxide layer and gaps inthe surrounding impervious confining material can be tolerated. FIGS. 9and 10 illustrate the excellent retention characteristics of the silicondioxide devices.

The hydrogenous ions present small capture cross-sections to electronsand, as such, enable these devices to be radiation tolerant. Furtherexperimentation may extend this property to the radiation hard level.FIGS. 11A and 11B compare, respectively, a typical device of thisinvention with a typical prior art nonvolatile memory on the basis ofmobile charge density vs. time of exposure to radiation, demonstratingthat the protons have much longer endurance than holes.

The above examples have discussed structures formed by SIMOX processingand conventional silicon MOS processing techniques. Other processes maybe employed to form the memory elements including but not limited tozone melt recrystallization (ZMR), UNIBOND® processing, and bonded andetchback silicon on insulator (BESOI).

Optical Memory

The above described electronic memory elements can also be used tocreate optical memory elements that are electrically written and erasedand optically read. The basic functional element of the dielectric layercontaining the mobile hydrogenous ions flanked by layers that confinethe ions remains and is combined with the added component of anoptically active layer that is sensitive to the proximity of theelectric field created by the mobile ions. This optical layer could be atransparent piezoelectric layer whose refractive index changes with theelectric field or a liquid crystal layer whose polarization changes withthe electric field or other materials with the same or other opticalproperties affected by the electric field of the mobile ions.

FIG. 12 is a view of a reflective embodiment of the optical memoryelement. The dielectric layer 50 is surrounded by a back side layer 52,typically a silicon substrate, and by a front capping layer 54,typically a thin silicon layer. The optically active layer 56 locatedbetween the front capping layer 54 and a transparent electrode layer 58,preferably tin oxide or indium tin oxide. The mobile hydrogenous ionsare introduced into the dielectric layer as discussed above and prior toformation of the optical layer 56. Light from an optical source 64passes through the first polarizer 62, enters the optical layer 56, herea liquid crystal with a polarizing effect upon the light. The lightleaves the element, and passes through another polarizer 62 to reach anoptical detector. If the mobile hydrogenous ions have been moved closeto the front capping layer 54, the liquid crystal layer 56 will be inone polarization state; and, if the ions have been moved away from thefront capping layer 56, the liquid crystal will be in anotherpolarization state. The particular polarization state the optical memoryelement is in will determine whether light transmitted from the source64 will reach the detector 66. If the optical layer were a piezoelectricmaterial, the polarizers 62 would not be needed. Instead, the refractiveindex of the optical layer 56 would change and the exit beam would beangularly displaced either onto or away from the fixed detector in thisalternate embodiment.

FIG. 13 is a view of a transmissive embodiment of the optical memoryelement. The differences between this embodiment and the embodiment ofFIG. 12 are that there is an additional transparent substrate layer 53typically made of quartz or glass, with a thin silicon layer 55 servingas a rear capping layer to confine the mobile hydrogenous ions on theback side of the dielectric layer 50, here silicon dioxide. As this is atransmissive element, the optical source 64 has been moved to the backside of the element along with one of the polarizers 62. The operationis otherwise similar to that discussed above for the liquid crystalembodiment and for the piezoelectric material embodiment.

If the individual optical memory elements were combined into a largearray with individually electrically written elements, the array wouldbecome a large scale memory array or, with suitable redesign, the basisfor a flat panel display.

We claim:
 1. A memory element comprising:a first dielectric layercontaining mobile hydrogenous ions which create an electric field; alower layer below the first layer of a material highly impervious tomigration of hydrogenous ions therethrough; an upper layer above thefirst layer of a material highly impervious to migration of hydrogenousions therethrough; and means to change the spatial position of thehydrogenous ions and the electric field created thereby within the firstlayer.
 2. The memory element of claim 1 additionally comprising means tosense the position of the electric field with the first layer.
 3. Thememory element of claim 2 wherein the means to sense the position of theelectric field comprises means to sense the conductivity of either theupper or lower layer adjacent to the first layer.
 4. The memory elementof claim 2 wherein the means to sense the position of the electric fieldcomprises means to sense an optical property in an additional layer of amaterial that exhibits a change in the optical property in response to achange in the proximity of the electric field in the first layer.
 5. Amicroelectronic memory element comprising:a silicon substrate layer; afirst layer containing mobile hydrogenous ions; a second silicon layerabove the first layer; and means to change the conductivity of at leastone of the silicon layers by changing the spatial position of thehydrogenous ions in the first layer.
 6. The memory element of claim 5wherein the first layer comprises a dielectric.
 7. The memory element ofclaim 6 wherein the dielectric is selected from the group consisting ofsilicon oxide, barium strontium titanate, tantalum oxide and nitridedoxide.
 8. The memory element of claim 5 additionally comprising materialhighly impervious to migration of hydrogenous ions therethroughsurrounding the first layer.
 9. The memory element of claim 8 whereinthe material is selected from the group consisting of nitrides, nitridedoxide, silicon nitride, polysilicon and monocrystalline silicon.
 10. Thememory element of claim 5 further comprising interconnections to othernon-memory microelectronic elements formed on the same siliconsubstrate.
 11. The memory element of claim 5 further comprising crackingsites located in the first layer for creation of the hydrogenous ionsfrom the cracking of the molecules of a hydrogenous gas.
 12. The memoryelement of claim 5 wherein the hydrogenous ions are selected from thegroup consisting of hydrogen ions, deuterium ions, tritium ions, andcombinations thereof.
 13. The memory element of claim 5 wherein thememory element is non-volatile.
 14. The memory element of claim 5wherein the means to change conductivity comprises substrate electrodemeans and gate electrode means, said gate electrode means being locatedabove the second silicon layer, with the substrate electrode means andthe gate electrode means being connected to reversible voltage biasmeans which drives the hydrogenous ions towards the upper surface of thefirst layer when the substrate electrode means is biased positivelyrelative to the gate electrode means and drives the hydrogenous ionstowards the lower surface of the first layer when the gate electrodemeans is biased positively relative to the substrate electrode means.15. The memory element of claim 5 further comprising an intermediatesilicon layer immediately above the first layer and an additional layercontaining mobile hydrogenous ions above the intermediate silicon layerand below the second silicon layer and an additional component in themeans to change the conductivity of the second silicon layer to changethe spatial position of the hydrogenous ions in the additional layer toform a multi-state memory element.
 16. An optical memory elementcomprising:a first dielectric layer containing mobile hydrogenous ions;a lower layer, below the first layer, of a material highly impervious tomigration of hydrogenous ions therethrough; an upper layer, above thefirst layer, of a material highly impervious to migration of hydrogenousions therethrough; an electro-sensitive optical layer above the upperlayer in which an optical property changes in response to a change inthe spatial position of the hydrogenous ions in the first layer; and anelectrode means associated with the lower layer and a transparentelectrode means located above the electro-sensitive optical layer, withboth electrode means connected to reversible voltage bias means tochange the spatial position of the hydrogenous ions within the firstlayer.
 17. The optical memory element of claim 16 wherein the opticalproperty is polarization.
 18. The optical memory element of claim 16wherein the optical property is absorption.
 19. The optical memoryelement of claim 16 wherein the element is transparent.